Secretion signal peptides assist proteins in traversing cell membranes. They are used by both prokaryotes, such as E. coli, and eukaryotic cells. The vast majority of secretory proteins do not share a uniform consensus sequence (Watson, Nucl. Acids. Res. 12:5145 (1984)). In fact they are quite divergent. Substitutions between signal sequences of different species is highly unpredictable and usually results in inconsistent secretion between species, e.g., prokaryotes and eukaryotes. Few signal sequences result in efficient secretion in both prokaryotes and eukaryotes.
There is a significant interest in producing recombinant proteins in a secreted form. Secretion allows for easy recovery of the recombinant protein of interest from the medium in which the host cell is grown.